208
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
by Kelvin-Helmholtz instabilities that generate large
coherent vortices at the interface between the canopy
and the overlaying water column, where the velocity
profiles exhibit an inflection point. These vortices
can penetrate into the canopy (Ghisalberti and Nepf,
2002) and interact with the flexible and buoyant
plants in a hydroelastic response due to the buoyancy
created by gas-filled lacunae and propagate downstream over the canopy causing the shoots to wave
in a coherent manner (Ghisalberti and Nepf, 2002).
This synchronous motion of seagrasses results in enhanced vertical transport between the water column
and the canopy (Ghisalberti and Nepf, 2002). The
ecological consequence of monamis is the potential
to increase larval recruitment (Grizzle et al., 1996)
and nutrient uptake.
D. Water Flow and Nutrient Uptake
in Seagrass Canopies
Nutrient uptake at the canopy level is a function of
water velocity. Increasing velocities lead to higher
uptake of ammonium in Thalassia testudinum and
Halodule wrightii (Thomas et al., 2000) and their
epiphytes (Cornelisen and Thomas, 2002). At the
same time, as velocity increases, the leaves bend, decreasing the obstruction/friction of the canopy. As a
result, the efficiency of the canopy to remove ammonium from the water column decreases at high
velocities (Thomas et al., 2000), unless leaf flapping
due to monamis or the orbital motion of waves cause
mixing within the canopy (Wallace and Cox, 1997;
Koch and Gust, 1999; Ghisalberti and Nepf, 2002).
The depth that the upper, highly turbulent flow at
the top of the canopy (with relatively high nutrient
levels) penetrates into the canopy is a function of
the percent of the water column that is occupied by
the canopy and the density of the shoots (Nepf and
Vivoni, 2000). Dense canopies occupying most of
the water column have narrow zones of high turbulence “skimming flow” at the top of their canopies.
Therefore, the flux of nutrients to areas deep within
the canopy may be limited.
Mixing and vertical exchange between the water
column and the seagrass canopy are a function of the
prevailing hydrodynamic conditions and also of how
seagrasses respond to them by bending, flexing, waving, etc. When leaves bend under strong currents, the
canopy height decreases collapsing the leaves onto
each other, “closing” the canopy (Fig. 8A and B) and
limiting vertical exchange (Koch and Gust, 1999). In
contrast, when seagrasses are exposed to monamis in
unidirectional flows, or wave-dominated conditions,
the back and forth motion of the leaves (Fig. 8C, D,
and E) enhances vertical exchange (Koch and Gust,
1999; Ghisalberti and Nepf, 2002). This suggests
that nutrient uptake should be highest at the upper
portion of the canopy (Nepf and Vivoni, 2000), but
this is also where the oldest and least biologically
active portions of the seagrass leaves are located.
Perhaps the vertical mixing zone at the top of the
canopy is more important in the recruitment process
bringing larvae and spores into the canopy (Grizzle
et al., 1996) than in the flux of nutrient and carbon
molecules.
E. Faunal Recruitment in Seagrass Canopies
The ecological role of seagrass canopies in the ecology of benthic and pelagic organisms is becoming increasingly clear, especially in the case of crustaceans
and fish (e.g. Kenyon et al., 1999; Thayer et al., 1999;
Etherington and Eggleston, 2000; Nagelkerken et al.,
2001). Many of these organisms are ecologically and
economically important species that settle in or on
the leaves and shoots of seagrasses as epiphytes for a
portion of their life history (Eckman, 1987; Borowitzka and Lethbridge, 1989; Newell et al., 1991;
Grizzle et al., 1996). Larval settlement appears to
be a function of larval supply (i.e. flux), the fluid
dynamic interaction with boundaries on which settlement occurs, and larval behavior (e.g. Abelson
and Denny, 1997; Okubo et al., 2002). As indicated
above, flows through vegetated areas are quite complex, and consequently, a mechanistic understanding
of faunal recruitment in seagrass canopies is lacking
at present, although recent efforts have been directed
to these ends (e.g. Palmer et al., 2004). Fortunately,
there are a number of processes and taxonomic systems including sediment dynamics in which the role
of canopy flow has been examined.
A recent estimate from the field indicates that the
potential for particle contact with a leaf surface approaches certainty under particular flow conditions
in a Zostera marina canopy (Ackerman, 2002; see
Section IV.D). The situation is more complex in
terms of larval settlement. The general pattern that
emerges is that settlement is higher in vegetated areas
in the case of bivalve larvae (e.g. scallops) settling
on Z. marina leaves (Eckman, 1987) and filamentous benthic algae (Harvey et al., 1995). For example, blue mussel recruitment on Z. marina leaves can
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
by Kelvin-Helmholtz instabilities that generate large
coherent vortices at the interface between the canopy
and the overlaying water column, where the velocity
profiles exhibit an inflection point. These vortices
can penetrate into the canopy (Ghisalberti and Nepf,
2002) and interact with the flexible and buoyant
plants in a hydroelastic response due to the buoyancy
created by gas-filled lacunae and propagate downstream over the canopy causing the shoots to wave
in a coherent manner (Ghisalberti and Nepf, 2002).
This synchronous motion of seagrasses results in enhanced vertical transport between the water column
and the canopy (Ghisalberti and Nepf, 2002). The
ecological consequence of monamis is the potential
to increase larval recruitment (Grizzle et al., 1996)
and nutrient uptake.
D. Water Flow and Nutrient Uptake
in Seagrass Canopies
Nutrient uptake at the canopy level is a function of
water velocity. Increasing velocities lead to higher
uptake of ammonium in Thalassia testudinum and
Halodule wrightii (Thomas et al., 2000) and their
epiphytes (Cornelisen and Thomas, 2002). At the
same time, as velocity increases, the leaves bend, decreasing the obstruction/friction of the canopy. As a
result, the efficiency of the canopy to remove ammonium from the water column decreases at high
velocities (Thomas et al., 2000), unless leaf flapping
due to monamis or the orbital motion of waves cause
mixing within the canopy (Wallace and Cox, 1997;
Koch and Gust, 1999; Ghisalberti and Nepf, 2002).
The depth that the upper, highly turbulent flow at
the top of the canopy (with relatively high nutrient
levels) penetrates into the canopy is a function of
the percent of the water column that is occupied by
the canopy and the density of the shoots (Nepf and
Vivoni, 2000). Dense canopies occupying most of
the water column have narrow zones of high turbulence “skimming flow” at the top of their canopies.
Therefore, the flux of nutrients to areas deep within
the canopy may be limited.
Mixing and vertical exchange between the water
column and the seagrass canopy are a function of the
prevailing hydrodynamic conditions and also of how
seagrasses respond to them by bending, flexing, waving, etc. When leaves bend under strong currents, the
canopy height decreases collapsing the leaves onto
each other, “closing” the canopy (Fig. 8A and B) and
limiting vertical exchange (Koch and Gust, 1999). In
contrast, when seagrasses are exposed to monamis in
unidirectional flows, or wave-dominated conditions,
the back and forth motion of the leaves (Fig. 8C, D,
and E) enhances vertical exchange (Koch and Gust,
1999; Ghisalberti and Nepf, 2002). This suggests
that nutrient uptake should be highest at the upper
portion of the canopy (Nepf and Vivoni, 2000), but
this is also where the oldest and least biologically
active portions of the seagrass leaves are located.
Perhaps the vertical mixing zone at the top of the
canopy is more important in the recruitment process
bringing larvae and spores into the canopy (Grizzle
et al., 1996) than in the flux of nutrient and carbon
molecules.
E. Faunal Recruitment in Seagrass Canopies
The ecological role of seagrass canopies in the ecology of benthic and pelagic organisms is becoming increasingly clear, especially in the case of crustaceans
and fish (e.g. Kenyon et al., 1999; Thayer et al., 1999;
Etherington and Eggleston, 2000; Nagelkerken et al.,
2001). Many of these organisms are ecologically and
economically important species that settle in or on
the leaves and shoots of seagrasses as epiphytes for a
portion of their life history (Eckman, 1987; Borowitzka and Lethbridge, 1989; Newell et al., 1991;
Grizzle et al., 1996). Larval settlement appears to
be a function of larval supply (i.e. flux), the fluid
dynamic interaction with boundaries on which settlement occurs, and larval behavior (e.g. Abelson
and Denny, 1997; Okubo et al., 2002). As indicated
above, flows through vegetated areas are quite complex, and consequently, a mechanistic understanding
of faunal recruitment in seagrass canopies is lacking
at present, although recent efforts have been directed
to these ends (e.g. Palmer et al., 2004). Fortunately,
there are a number of processes and taxonomic systems including sediment dynamics in which the role
of canopy flow has been examined.
A recent estimate from the field indicates that the
potential for particle contact with a leaf surface approaches certainty under particular flow conditions
in a Zostera marina canopy (Ackerman, 2002; see
Section IV.D). The situation is more complex in
terms of larval settlement. The general pattern that
emerges is that settlement is higher in vegetated areas
in the case of bivalve larvae (e.g. scallops) settling
on Z. marina leaves (Eckman, 1987) and filamentous benthic algae (Harvey et al., 1995). For example, blue mussel recruitment on Z. marina leaves can